The present invention generally relates to user equipment (UE) synchronization to a base station. More particularly the present invention relates to a cell search system which utilizes an improved initial cell search algorithm.
Initial cell search algorithms are used to synchronize the UE to a base station. The UE accomplishes this procedure via a common downlink channel called the physical synchronization channel (PSCH). Referring to FIG. 2, the PSCH has a structure wherein the same primary synchronization code (PSC) is transmitted at the beginning of each slot, while a secondary synchronization code (SSC) is transmitted for each slot, resulting in fifteen (15) different SSCs. As those skilled in the art know, a frame that is fifteen (15) slots long can transmit fifteen (15) SSCs.
The transmit order of the SSC depends on the primary scrambling code group number. As an example, in a five hundred and twelve (512) cell system, there are sixty four (64) groups. In each group, the patterns of the SSC and its cyclic shifts are different. As a result, there are five hundred and twelve (512) primary scrambling codes. Each cell, of a five hundred and twelve cell (512) system, is assigned a code such that no one code is used by more than one cell in a given reception area.
Therefore the cell search synchronization systems determine the primary scrambling code of a cell utilizing an initial cell search algorithm. Common initial cell search algorithms utilize three (3) major algorithms: a step 1 algorithm detects the PSC and determines a chip offset; a step 2 algorithm uses the information given by step 1 and detects the slot offset and code group number; and a step 3 algorithm utilizes the information provided by the step 2 algorithm and detects the primary scrambling code. Unfortunately, each step algorithm has an inherent error associated with it. The error present in each of the steps is caused by the UE detection of noise associated with the received common downlink channel, which can result in a high number of false detections.
Also, the common initial cell search algorithms can not handle a rejection by the upper layers of the wrong public land mobile network (PLMN). Since most algorithms detect the strongest cell in the common downlink channel, it is likely that each time the algorithm locates a cell the same PLMN will be associated with the cell. This results in a deadlock and ultimately an indication to the UE that there is no service.
Accordingly, there exists a need for a system and method that reduces the number of false detections by the initial cell search algorithm and is able to overcome the deadlock associated with a rejection due to the wrong PLMN.
The present invention is a system and method for conducting initial cell search and establishing a communication link between a UE and a base station in a communication network. The communication signal is first processed to generate an index value and chip offset associated with a primary synchronization code, producing a first decision. A peak sample of the communication signal is extracted and a second processing is conducted on the communication signal. The second processing retrieves the code group number, slot offset, and secondary synchronization code from the communication signal, producing a second decision. A third processing is conducted on the communication signal in response to the code group number and slot offset, which retrieves the primary scrambling code. The primary scrambling code is then used for synchronizing the UE to the cell associated with the primary scrambling code. Throughout the process, window exclusion logic is employed to enhance the screening of frequency bands while performing the initial code search. Through a system of buffers and counters, rejected chip offsets and rejected primary scrambling codes are stored for the purpose of checking and comparing by the window exclusion logic, which can then restart the decision process upon detection of a wrong PLMN, thereby avoiding a deadlock condition.